An early response to tissue hypoxia is induction of hypoxia inducible factor (HIF), a basic helix-loop-helix (bHLH) PAS (Per/Arnt/Sim) transcriptional activator that mediates changes in gene expression in response to changes in cellular oxygen concentration. HIF is a heterodimer containing an oxygen-regulated alpha subunit (HIFα) and a constitutively expressed beta subunit (HIFβ), also known as aryl hydrocarbon receptor nuclear transporter (ARNT). In oxygenated (normoxic) cells, HIFα subunits are rapidly degraded by a mechanism that involves ubiquitination by the von Hippel-Lindau tumor suppressor (pVHL) E3 ligase complex. Under hypoxic conditions, HIFα is not degraded, and an active HIFα/β complex accumulates in the nucleus and activates the expression of several genes including glycolytic enzymes, glucose transporter (GLUT)-1, erythropoietin (EPO), and vascular endothelial growth factor (VEGF). (Jiang et al. (1996) J Biol Chem 271:17771-17778; Iliopoulus et al. (1996) Proc Natl Acad Sci USA 93:10595-10599; Maxwell et al. (1999) Nature 399:271-275; Sutter et al. (2000) Proc Natl Acad Sci USA 97:4748-4753; Cockman et al. (2000) J Biol Chem 275:25733-25741; and Tanimoto et al. (2000) EMBO J. 19:4298-4309.)
Levels of HIFα protein are elevated in most cells in response to hypoxia and HIFα is induced in vivo when animals are subjected to anemia or hypoxia. HIFα levels rise within a few hours after the onset of hypoxia and return to baseline under continued hypoxic conditions. HIF has been implicated in numerous cellular and developmental processes including cell proliferation, angiogenesis, and cell cycle arrest. HIFα has also been associated with myocardial acute ischemia and early infarction, pulmonary hypertension, and inflammation. Although HIFα has been associated with tumor growth and metastasis, there is little indication that HIF is directly involved in tumorigenesis. Hypoxic preconditioning, in which a target organ is subjected to brief periods of hypoxia, has been shown to protect both myocardium and brain against hypoxic-ischemic injury. HIFα stabilization is closely associated with ischemia and is induced by preconditioning. (Wang and Semenza (1993) Proc Natl Acad Sci USA 90:4304-4308; Stroka et al. (2001) FASEB J 15:2445-2453; Semenza et al. (1997) Kidney Int 51:553-555; Carmeliet et al. (1998) Nature 394:485-490; Zhong et al. (1999) Cancer Res 59:5830-5835; Lee et al. (2000) N Engl J Med 343:148-149; Sharp et al. (2000) J Cereb Blood Flow Metab 20:1011-1032; Semenza et al. (2000) Adv Exp Med Biol 475:123-130; Thornton et al. (2000) Biochem J 350:307-312; Deindl and Schaper (1998) Mol Cell Biochem 186:43-51; Bergeron et al. (2000) Ann Neurol 48:285-296.)
Several investigators have studied the mechanism of interaction between HIFα and pVHL. An oxygen-dependent degradation domain (ODD) within HIF-1α from residue 401 to 603 was originally identified as sufficient to confer oxygen-dependent instability to chimeric protein constructs. A domain containing a portion of the ODD, from residue 526 to 652, was found to be required for pVHL-dependent degradation. Further, mutation of P564YI to aspartic acids or mutation of K532 to arginine within a region conserved among HIFα homologs (residue 556 to 574 in HIF-1α) rendered the full-length HIFα protein stable under normoxic conditions and resistant to pVHL-mediated degradation. (Huang et al. (1998) Proc Natl Acad Sci USA 95:7987-7992; and Tanimoto et al. (2000) EMBO J. 19:4298-4309.)
HIFα levels are increased by a number of factors that mimic hypoxia, including iron chelators such as desferrioxamine (DFO) and divalent metal salts such as CoCl2 HIFα levels are increased by angiotensin II, thrombin, and platelet-derived growth factor under normoxic conditions using a mechanism involving reactive oxygen species. Reports have also suggested HIFα is regulated by phosphorylation through pathways involving nitric oxide-activated phosphotidylinositol 3′-kinase (PI3K), hepatocyte growth factor, or mitogen-activated protein kinase. Glycogen-synthase kinase, which is a downstream target of PI3K, directly phosphorylates the HIFα ODD domain. (Richard et al. (2000) J Biol Chem 275:26765-26771; Sandau et al. (2000) Biochem Biophys Res Commun 278:263-267; Tacchini et al. (2001) Carcinogenesis. 22:1363-1371; and Sodhi et al. (2001) Biochem Biophys Res Commun 287:292-300.)
Hypoxia, a state of reduced oxygen, can occur when the lungs are compromised or blood flow is reduced. Ischemia, reduction in blood flow, can be caused by the obstruction of an artery or vein by a blood clot (thrombus) or by any foreign circulating matter (embolus), or by a vascular disorder such as atherosclerosis. Reduction in blood flow can have a sudden onset and short duration (acute ischemia), or can have a slow onset with long duration or frequent recurrence (chronic ischemia). Acute ischemia is often associated with regional, irreversible tissue necrosis (an infarct), whereas chronic ischemia is usually associated with transient hypoxic tissue injury. If the decrease in perfusion is prolonged or severe, however, chronic ischemia can also be associated with an infarct. Infarctions commonly occur in the spleen, kidney, lungs, brain, and heart, producing disorders such as intestinal infarction, pulmonary infarction, ischemic stroke, and myocardial infarction.
Pathologic changes in ischemic disorders depend on the duration and severity of ischemia, and on the length of patient survival. Necrosis can be seen within the infarct in the first 24 hours, and an acute inflammatory response develops in the viable tissue adjacent to the infarct with leukocytes migrating into the area of dead tissue. Over succeeding days, there is a gradual breakdown and removal of cells within the infarct by phagocytosis, and replacement with a collagenous or glial scar.
Hypoperfusion or infarction in one organ often affects other organs. For example, ischemia of the lung, caused by, for example, a pulmonary embolism, not only affects the lung, but also puts the heart and other organs, such as the brain, under hypoxic stress. Myocardial infarction, which often involves coronary artery blockage due to thrombosis, arterial wall vasospasms, or viral infection of the heart, can lead to congestive heart failure and systemic hypotension. Secondary complications such as global ischemic encephalopathy can develop if the cardiac arrest is prolonged with continued hypoperfusion. Cerebral ischemia, most commonly caused by vascular occlusion due to atherosclerosis, can range in severity from transient ischemic attacks (TIAs) to cerebral infarction or stroke. While the symptoms of TIAs are temporary and reversible, TIAs tend to recur and are often followed by a stroke.
Occlusive arterial disease includes coronary artery disease, which can lead to myocardial infarction, and peripheral arterial disease, which can affect the abdominal aorta, its major branches, and arteries of the legs. Peripheral arterial disease includes Buerger's disease, Raynaud's disease, and acrocyanosis. Although peripheral arterial disease is commonly caused by atherosclerosis, other major causes include, e.g., diabetes, etc. Complications associated with peripheral arterial disease include severe leg cramps, angina, abnormal heart rhythms, heart failure, heart attack, stroke, and kidney failure.
Ischemic and hypoxic disorders are a major cause of morbidity and mortality. Cardiovascular diseases cause at least 15 million deaths every year and are responsible for 30% of deaths worldwide. Among the various cardiovascular diseases, ischemic heart disease and cerebrovascular diseases cause approximately 17% of deaths. Annually, 1.3 million cases of nonfatal acute myocardial infarction are reported, making the prevalence approximately 600 per 100,000 people. Further, an estimated five million Americans suffer from venous thrombosis every year, and approximately 600,000 of these cases result in pulmonary embolism. About one-third of the pulmonary embolisms end in death, making pulmonary embolism the third most common cause of death in the United States.
Currently, treatment of ischemic and hypoxic disorders is focused on relief of symptoms and treatment of causative disorders. For example, treatments for myocardial infarction include nitroglycerin and analgesics to control pain and relieve the workload of the heart. Other medications, including digoxin, diuretics, amrinone, β-blockers, lipid-lowering agents and angiotensin-converting enzyme inhibitors, are used to stabilize the condition, but none of these therapies directly address the tissue damage produced by the ischemia and hypoxia.
Due to deficiencies in current treatments, there remains a need for methods that are effective in treating conditions involving ischemia and hypoxia such as occlusive arterial disease, angina pectoris, intestinal infarctions, pulmonary infarctions, cerebral ischemia, and myocardial infarction. There is also a need for methods that are effective in the prevention of tissue damage caused by ischemia that occurs due to, e.g., atherosclerosis, diabetes, and pulmonary disorders such as pulmonary embolism and the like. In summary, there is a need in the art for methods and compounds that can be used to stabilize HIF, and to treat and prevent HIF-associated disorders including conditions involving ischemia and hypoxia.